A major problem for immunological approaches to the control of HIV is the extreme variability of the viral genome, which is reflected in a corresponding antigenic variability. This problem has hampered attempts to design effective vaccines as well as attempts to develop immunotherapies. It is, thus, well recognized that the identification of neutralizing but non-variable epitopes would constitute a major advance in this area.
The HIV envelope is composed of two glycoproteins, gp120 and gp41. These glycoproteins are initially synthesized in virus-infected cells as a precursor called gp160; this molecule is cleaved into gp120 and gp41 prior to assembly of virions. The latter two glycoproteins are non-covalently associated with each other and are anchored to the viral membrane via gp41, a transmembrane protein (reviewed in (Olshevsky et al. 1990)).
One region which has been shown to elicit neutralizing antibodies is the V3 region hypervariable loop (hvl-V3) of the gp120 (amino acids 307-330); this is an immunodominant epitope cluster eliciting potent neutralizing Abs in man and experimental animals (summarized in (Javaherian et al. 1990)). Initially, there was the concern that the hypervariability of the V3 loop would prevent the design of a rational vaccine based on this epitope. However, LaRosa et al. (LaRosa et al. 1990) have recently shown that the V3 loop is less variable than originally thought, and, in addition, anti-V3 Abs with broader HIV strain specificity have been generated (Javaherian et al. 1990); these Abs recognize a conserved hexamer sequence (Gly Pro Gly Arg Ala Phe (SEQ ID NO:1) present at the tip of the loop. Three anti-V3 human monoclonal antibodies (HuMAbs) have been isolated by other investigators, and each is relatively strain-specific, recognizing only the MN strain of virus and closely related strains (Scott et al. 1990, Zolla-Pazner et al. 1990).
Another epitope cluster of HIV envelope that has been shown to elicit neutralizing antibodies is the CD4 binding site of gp120. Recent evidence indicates that the CD4 binding site is formed by non-contiguous protein loops from multiple regions of gp120 (Olshevsky et al. 1990). However, the precise structure of the CD4 binding site and its contact residues have yet to be defined. Neutralizing antibodies against this site have been raised in some rodents (Sun et al. 1989, Lasky et al. 1987, Berman et al. 1989) using either recombinant gp120 or linear peptides adjacent to one of the loops apparently forming the CD4 binding site. It was believed that humans do not produce Abs against the CD4 binding site, partially because no human serum Abs could be shown to bind to the linear peptides discussed above (Sun et al. 1989, Lasky et al. 1987). We and two other groups (Robinson et al. 1990, Ho et al. 1991, Posner et al. 1990) have isolated HuMAbs against conformational, rather than linear, epitopes mapping in the CD4 binding region. These HuMAbs have neutralizing activity against a variety of divergent HIV-1 strains and, therefore, recognize relatively conserved epitopes.
Earlier in the AIDS epidemic, there was skepticism about the protective function of neutralizing Abs against HIV, since such Abs could be found in seropositive individuals who went on to develop AIDS. Now it is understood that the titers of neutralizing Abs developed in humans during the course of HIV infection are generally not very high (Robert-Guroff et al. 1985, Weiss et al. 1985), that higher titers of certain anti-HIV Abs do correlate with a better prognosis (Robert-Guroff et al. 1985, Rook et al. 1987, Ljunggren et al. 1987, Ho et al. 1987, Devash et al. 1990), and that deleterious Abs against HIV that actually enhance viral infection may be present in seropositive individuals (Robinson et al. 1990, Homsy et al. 1988, Takeda et al. 1988, Jouault et al. 1989). Furthermore, recent studies demonstrate the protective effects of certain anti-HIV Abs in vivo. In one such study, passive administration of hyperimmune plasma from healthy HIV-infected humans to ARC and AIDS patients resulted in sustained clearance of p24 antigen and a maintenance or increase in the recipients' anti-viral Ab titer, and clinical improvement was noted in 5 of 9 recipients (Karpas et al. 1988). In another study, chimpanzees were challenged with a stock of the IIIB strain of HIV that had previously been incubated with neutralizing serum Ab from an HIV-seropositive chimpanzee. The challenged animals were protected against viral infection, as assessed by lack of serum Ab response to virus and attempts at viral isolation (Emini et al. 1990). Very recently, successful long term protection of two chimpanzees against HIV infection has been demonstrated by immunization with recombinant gp160 followed by a V3 loop peptide (Girard et al. 1991). In a different study, chimpanzees immunized with recombinant gp120 and challenged with HIV were also protected from infection (Berman et al. 1990). In both of these vaccine trials, significant titers of strain-specific neutralizing Ab were induced prior to challenge with virus. The protection obtained is believed to be due primarily to this neutralizing Ab, since subunit vaccines are thought to be poor inducers of cytotoxic T cells (see (Berman et al. 1990)).
Viral neutralization by combinations of rodent mabs has been described for certain non-AIDS viruses, including rubella (Gerna et al. 1987), vesicular stomatitis (Volk et al. 1982), West Nile (Peiris et al. 1982), Sindbis (Clegg et al. 1983), Japanese encephalitis (Kimura-Kuroda and Yasui 1983), La Crosse (Kingsford 1984), Newcastle disease (Russell 1986), respiratory syncytial (Anderson et al. 1988), and bovine herpesvirus type 4 (Dubuisson et al. 1990) viruses. In these studies, relatively high levels of viral neutralization are attained by relatively low concentrations of two or more mAbs in combination than is attained by any of the mAbs alone.
To our knowledge, however, improved neutralization of HIV by a combination of Abs has not been reported, nor has anyone previously demonstrated synergistic neutralization of any virus by human mAbs.